Three-quarters of the way into “The Substance,” something truly wild happens.

Yes, OK — wild things start happening almost right away in the explosively gross horror-comedy that swept the nation this fall. But at this particular moment, the protagonist at last comes face to face with herself.

Our damaged heroine is 50-something Elisabeth Sparkle (Demi Moore), a once-celebrated actress turned television aerobics coach. After being summarily fired from her gig for, essentially, being too old, she found herself with a mysterious box containing a neon green substance in a hypodermic needle. That injection literally divided her into two bodies: the older one, and a younger, nubile one named Sue (Margaret Qualley), who got the job hosting Elisabeth’s old aerobics show and has rocketed onto the celebrity A-list.

The system is simple. Elisabeth and Sue are supposed to switch out every week — only one can be active in the world at a time, while the other has to spend a week in a kind of coma, regenerating spinal fluid. But Sue, loving her success, has instead kept Elisabeth in her coma state for months, slowly sucking the life out of her. (If the mechanics of all this don’t make a ton of sense, don’t sweat it — it’s the concept that’s important.) Elisabeth has aged much faster than natural as a result, and she is furious at Sue.

The first eruption came as a shock. But today, lava regularly snaking across the landscape is the new normal. “This was so strange at the beginning,” said Rebekka Hlin Runarsdottir, a geologist and technician at the University of Iceland. “And now, we’re just living in this reality.”

For over a century, these depths were scoured by miners in search of gold. Now they hold the Sanford Underground Research Facility, or SURF. In the coming years, some of the world’s top particle physicists plan to transform this realm into the listening end of an 800-mile, $5 billion tin-can telephone. With it, they hope to hear a whispered answer to an existential question: How did we get here?

By the time we get to this scene, Elisabeth’s entire body is wrinkled and decrepit, as if she’s 200 years old, and most of her hair has fallen out. Now she’s decided to terminate the whole thing, to inject Sue with a fluid that will essentially kill her off and leave Elisabeth to live out the rest of whatever life she has left. Sue is unconscious, and Elisabeth is hovering over her, ready to plunge the termination needle into her alternate self.

But she can’t quite do it. The two share a consciousness, so Elisabeth suddenly sees a few of Sue’s memories alongside her own — memories of being loved and adored, of being a success, of being famous. She pulls out the needle just before the deed can be completed.

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